The present invention relates generally to the field of bonding of substrates. In particular, the invention relates to methods of bonding substrates in MEMS and other devices to reduce the reflection loss of light.
MEMS and other devices often include two or more substrates either in close proximity or bonded together. For example, in optical systems such as digital projectors, a device may include an interference-based digital light display (DLD) package which includes two or more substrates to direct light to and from the DLD. Similar to a CRT in a rear-projection television, a DLD can be used in digital projectors for processing or generating an image from a source light.
One such DLD package is illustrated in FIG. 1. The package 100 includes a base substrate 120 with a driving electrode 122, a pixel plate 110 which can move vertically, and a thin protective substrate or membrane 130. A reflective coating may be provided on the pixel plate 110, and a partial reflective coating may be provided on the bottom surface of the membrane 130. The protective membrane 130 encloses a cavity in which the DLD pixel plate 110 is enclosed and allows some light to pass therethrough. In some cases, such as in the case of the package 100 illustrated in FIG. 1, a second substrate 140, which may be made of thick glass, is provided in close proximity to the protective membrane 130 for processing the light, for example. The protective membrane 130 and the second substrate 140 are separated by a bond ring 150 positioned at the perimeter of the protective membrane 130 and the second substrate 140. Thus, light 160 from a source (not shown) can pass through the second substrate 140 and the protective membrane 130 to reach the pixel plate 110. By moving the pixel plate vertically, different light colors are generated as a result of light interference. As the gap between the pixel plate 110 and the protective membrane 130 changes, the processed light 170 (image) goes through the second substrate 140 to additional light processing components such as lenses, for example.
As the light 160, 170 passes through the protective membrane 130 and the second substrate 140, it passes through regions of differing refractive indices (RI's). For example, the protective membrane 130 and the second substrate 140 may each have a different RI, while the space between the protective membrane 130 and the second substrate 140 may have a third different RI. This change in RI along the path of the light causes a portion of the light 160, 170 to be lost as reflected light 180, thereby reducing the quality of the image generated by the DLD package 100. To counter the reflection, the various surfaces of the protective membrane 130 and the second substrate 140 may be provided with anti-reflective coating. Such coating can be expensive and difficult to implement, particularly for certain internal surfaces. Further, the package may not be hermetically sealed, as moisture or gas molecules may penetrate the bond ring 150.